The magnetic amplifier post regulator is a popular way to regulate the auxiliary outputs of a switched-mode power supply (SMPS). The SMPS couples power from a primary circuit to a main output circuit and an auxiliary output circuit through a transformer. A pulse width modulator and a power switch transistor control the transfer of power from the primary circuit. Therefore, the SMPS produces a main output voltage V.sub.OM and an auxiliary output voltage V.sub.OS. Magnetic amplifiers provide a simple, efficient, and reliable way of providing precise voltage regulation of independent outputs of a multiple output SMPS. The magnetic amplifier post regulator includes a magnetic amplifier, a reset transistor, and an error amplifier.
The error amplifier generates an error signal in response to sensing the auxiliary output circuit voltage, V.sub.OS. The error amplifier produces the error signal to control the blocking time of the magnetic amplifier by providing the appropriate reset of the magnetic amplifier core flux during T.sub.OFF of the power switch transistor. If the output voltage V.sub.OS is too high, the error amplifier turns the reset transistor on harder to supply more reset current into the magnetic amplifier in the reverse direction. Increasing the reset of the magnetic flux causes the blocking time of the magnetic amplifier to increase. This causes V.sub.OS to decrease. Similarly, if V.sub.OS is too low, the error amplifier causes the reset current to decrease, which in turn causes V.sub.OS to increase. This current reset technique then resets the magnetic amplifier core to a level that yields the correct reset time for the desired V.sub.OS.
The magnetic amplifier core may also be reset using a voltage reset technique. In the voltage reset technique, voltage clamping is applied at the output side of the magnetic amplifier. The current reset technique is more popular because of its ease of implementation and because it exhibits lower phase lag at high frequencies than does the voltage reset technique.
The reset transistor is commonly referenced to the output when output regulation must extend to zero load current. This prevents the magnetizing current of the magnetic amplifier from raising the output under no-load conditions.
In many instances it is necessary that the voltage from the magnetic amplifier post regulator circuit equal zero. One example of such a requirement exists when it is necessary to provide output current limiting when the output is short-circuited. However, for complete shutdown to occur, the magnetic amplifier blocking time, T.sub.B, must equal the primary circuit power switch transistor on-time, T.sub.ON. When the reset transistor is referenced to the output, the reset transistor has insufficient compliance voltage to provide the necessary reset current to achieve full shutdown of the magnetic amplifier.
There is a need, therefore, for a method and circuitry for providing complete shutdown of the magnetic amplifier of a magnetic amplifier post regulator which has its reset transistor referenced to the output.